The obligate intracellular pathogen, Coxiella burnetii, causes acute and chronic disease in humans. These acidophilic rickettsia are phagocytized by a broad array of host cell types, becoming metabolically active and replicating solely within the phagolysosome. An integrated model for pathogenic mechanisms used by different strains to cause either acute or chronic disease will be tested. In this model, C. burnetii invade a wide range of host cells by recognizing ubiquitous receptors. These organisms must employ unique survival mechanisms to replicate in the phagolysosome. Chronic strains persist by resisting the host response, which includes the induction of inflammatory cytokines, causing chronic tissue damage at sites of vegetative growth. Only limited studies have been done to characterize the major surface proteins that are likely involved in pathogenic mechanisms. The focus of this proposal is to genetically and antigenically define surface proteins from acute and chronic strains of C.burnetii. Four membrane proteins, Com-1, P1, P2, and Com-2 will be cloned to determine the role of each protein in our model. Our studies showed Com-1 to be surface exposed and likely to be involved in catalyzing the formation of disulfide bonds. Disulfide cross linking of C. burnetii outer membrane proteins may allow survival in the harsh phagolysosomal environment. P1 is an integral membrane protein with homology to OmpA of Escherichia coli, and is therefore likely to function in membrane integrity, resistance to complement-mediated killing and as a porin. P2, a putative lipoprotein, may function as other bacterial lipoproteins to induce inflammatory cytokines. Com-2 was isolated from a C. burnetii gene, bank with monoclonal antibodies to a protein found in acute and chronic bacterial strains. Experiments will define the host receptor binding by acute and chronic strains, the surface proteins involved in this interaction, and the role of this binding in vivo. Survival and host response evasion may involve a developmentally regulated life cycle and studies will define which major surface proteins are expressed by each cell form. Finally, studies will determine the ability of C. burnetii to survive in the presence of interferon-gamma and the potential of individual membrane proteins to elicit inflammatory cytokines. These four proteins and other membrane proteins will be evaluated for their individual role in the three major facets of our model of pathogenesis. Defining these pathogenic mechanisms will allow a clearer understanding of how this disease paradigm relates to other intracellular bacterial diseases. These studies will also allow the prediction of new vaccination and therapeutic strategies.